A typical fiber optic transmission system is shown in FIG. 1 and includes an optical transmitter 1002, a transmission fiber 1003, and a receiver 1004.
In a digital communications application, a key figure of merit for a transmitter in a communications link is its bit error rate (BER) performance, which is measured by a standard receiver.
More particularly, in a systems test, a known pattern of digital 1 and 0 bits is generated by a pattern generator 1001, converted to optical digital signal 1006 by the transmitter 1002, and injected into the communications link, e.g., fiber 1003. The data coming out of transmitter 1002 is often shown on a sampling oscilloscope in the form of an “eye diagram” 1006, as illustrated in FIG. 1. The “eye” diagram is generated by superimposing the pulse train repeatedly on itself, each time shifting it by one bit period. The top “rail” represents the 1s and the bottom “rail” rail represents the 0 bits. The data coming out of fiber 1003 can be shown on a sampling oscilloscope in the form of another optical eye 1020, and may be distorted by fiber dispersion. The receiver 1004 converts the optical signal into a corresponding electrical signal and determines if the bits are 1s or 0s using a decision circuit, which distinguishes is as signals above a certain preset decision threshold and 0s as signals below the threshold.
In a systems test, an error detector 1005 counts errors: the number of 1s that were intended as 0s and vice versa per unit time. This is called the bit error rate (BER). The error rate is measured as a function of the received optical power into the receiver, since the error rate is a function of the noise in the receiver as well as distortions in the eye.
A transmitter is typically characterized by its BER without fiber transmission, the so-called back-back BER, and its BER after transmission. FIG. 2 shows a BER curve versus received power for a particular transmitter. In FIG. 2, the back-back BER is shown at 1020 and BER after transmission is shown at 1021. The power penalty is shown at 1022. As is typical, the bit error rates for both back-back and after transmission reduce with increasing optical power, since the signal-to-noise ratio increases with increasing optical power. The optical power at which a certain back-back BER (typically 10−12) is achieved is called the sensitivity 1023 and is determined by a transmitter-receiver pair. The sensitivity of a receiver is defined as the sensitivity achieved with an optimal transmitter, typically an externally modulated LiNbO3 transmitter, which produces well defined pulses with high contrast ratio and little distortion without transmission though fiber. The distortions caused by fiber dispersion degrade the transmitted BER 1021 (here after 96 km of standard fiber) and increase the received optical power required to achieve a 10−12 BER. The difference between the back-back sensitivity and the sensitivity after transmission is called the dispersion penalty and is measured in dB. In the present example, the dispersion penalty is 1.5 dB.
Both transmitter and receiver are optimized in order to reduce the dispersion penalty to a desired value. Telecommunication standards at present call for a dispersion penalty of <2 dB. If the back-back sensitivity of the transmitter is worse than the receiver sensitivity, there is an additional back-back penalty, which reduces the overall power budget. The optical power budget is the sum of the optical loss and dispersion penalties, as well as any margin that the system may impose.
It is, therefore, generally desirable to optimize a transmitter (in order to meet the desired power budget) by reducing both the back-back penalty and the dispersion penalty.
A system for long-reach lightwave data transmission through optical fibers has been described in U.S. patent application Ser. No. 10/289,944, filed Nov. 06, 2002 by Daniel Mahgerefteh et al. for POWER SOURCE FOR A DISPERSION COMPENSATION FIBER OPTIC SYSTEM, which patent application is hereby incorporated by reference. Azna LLC of Wilmington, Mass. sometimes refers to the transmitter apparatus of this patent application as a Chirp Managed Laser (CML™). In this system, a frequency modulated (FM) source is followed by an optical discriminator, also sometimes referred to as an optical spectrum reshaper (OSR), which converts frequency modulation into a substantially amplitude modulated (AM) signal and partially compensates for the dispersion in the transmission fiber.
Also, in U.S. Provisional Patent Application Ser. No. 60/629,741, filed Nov. 19, 2004 by Yasuhiro Matsui et al. for OPTICAL SYSTEM COMPRISING AN FM SOURCE AND A SPECTRAL RESHAPING ELEMENT, which patent application is hereby incorporated herein by reference, there is disclosed a CML™ system that can be adapted to transmit a digital signal across >200 km, at 10 Gb/s, in a standard fiber having a net dispersion of 3200 ps/nm. This is approximately twice as far as can be achieved using a standard external modulated transmitter. In this patent application, it is disclosed that the amplitude and frequency profile of the transmitted signal can be adjusted so as to reduce the BER after transmission through dispersive fiber.
It is an object of the present invention to further decrease the bit error rate of the transmitted signal after propagation through fiber by adjusting certain parameters of the receiver in conjunction with the parameters of the transmitter.